[core] Make the BackgroundScheduler a singleton

- Do not carry it over everywhere as parameter, it is a shared
  instance anyway and the lifecycle is pretty much the app lifecycle
  from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
  do tasks in the background, we don't make assumptions if it is a
  thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.

4 files changed, 34 insertions, 27 deletions

diff --git a/platform/default/src/mbgl/gl/headless_frontend.cpp b/platform/default/src/mbgl/gl/headless_frontend.cpp
index 37b0f91f32..71cd656e59 100644
--- a/platform/default/src/mbgl/gl/headless_frontend.cpp
+++ b/platform/default/src/mbgl/gl/headless_frontend.cpp
@@ -8,11 +8,11 @@
 
 namespace mbgl {
 
-HeadlessFrontend::HeadlessFrontend(float pixelRatio_, FileSource& fileSource, Scheduler& scheduler, const optional<std::string> programCacheDir, GLContextMode mode, const optional<std::string> localFontFamily)
-    : HeadlessFrontend({ 256, 256 }, pixelRatio_, fileSource, scheduler, programCacheDir, mode, localFontFamily) {
+HeadlessFrontend::HeadlessFrontend(float pixelRatio_, FileSource& fileSource, const optional<std::string> programCacheDir, GLContextMode mode, const optional<std::string> localFontFamily)
+    : HeadlessFrontend({ 256, 256 }, pixelRatio_, fileSource, programCacheDir, mode, localFontFamily) {
 }
 
-HeadlessFrontend::HeadlessFrontend(Size size_, float pixelRatio_, FileSource& fileSource, Scheduler& scheduler, const optional<std::string> programCacheDir, GLContextMode mode, const optional<std::string> localFontFamily)
+HeadlessFrontend::HeadlessFrontend(Size size_, float pixelRatio_, FileSource& fileSource, const optional<std::string> programCacheDir, GLContextMode mode, const optional<std::string> localFontFamily)
     : size(size_),
     pixelRatio(pixelRatio_),
     backend({ static_cast<uint32_t>(size.width * pixelRatio),
@@ -23,7 +23,7 @@ HeadlessFrontend::HeadlessFrontend(Size size_, float pixelRatio_, FileSource& fi
             renderer->render(*updateParameters);
         }
     }),
-    renderer(std::make_unique<Renderer>(backend, pixelRatio, fileSource, scheduler, mode, programCacheDir, localFontFamily)) {
+    renderer(std::make_unique<Renderer>(backend, pixelRatio, fileSource, mode, programCacheDir, localFontFamily)) {
 }
 
 HeadlessFrontend::~HeadlessFrontend() = default;
diff --git a/platform/default/src/mbgl/map/map_snapshotter.cpp b/platform/default/src/mbgl/map/map_snapshotter.cpp
index 56bc9bdb88..9532b01b7e 100644
--- a/platform/default/src/mbgl/map/map_snapshotter.cpp
+++ b/platform/default/src/mbgl/map/map_snapshotter.cpp
@@ -15,7 +15,6 @@ namespace mbgl {
 class MapSnapshotter::Impl {
 public:
     Impl(FileSource*,
-         std::shared_ptr<Scheduler>,
          const std::pair<bool, std::string> style,
          const Size&,
          const float pixelRatio,
@@ -42,13 +41,11 @@ public:
     void snapshot(ActorRef<MapSnapshotter::Callback>);
 
 private:
-    std::shared_ptr<Scheduler> scheduler;
     HeadlessFrontend frontend;
     Map map;
 };
 
 MapSnapshotter::Impl::Impl(FileSource* fileSource,
-           std::shared_ptr<Scheduler> scheduler_,
            const std::pair<bool, std::string> style,
            const Size& size,
            const float pixelRatio,
@@ -56,9 +53,8 @@ MapSnapshotter::Impl::Impl(FileSource* fileSource,
            const optional<LatLngBounds> region,
            const optional<std::string> programCacheDir,
            const optional<std::string> localFontFamily)
-    : scheduler(std::move(scheduler_))
-    , frontend(size, pixelRatio, *fileSource, *scheduler, programCacheDir, GLContextMode::Unique, localFontFamily)
-    , map(frontend, MapObserver::nullObserver(), size, pixelRatio, *fileSource, *scheduler, MapOptions().withMapMode(MapMode::Static)) {
+    , frontend(size, pixelRatio, programCacheDir, GLContextMode::Unique, localFontFamily)
+    , map(frontend, MapObserver::nullObserver(), size, pixelRatio, *fileSource, MapOptions().withMapMode(MapMode::Static)) {
 
     if (style.first) {
         map.getStyle().loadJSON(style.second);
@@ -165,7 +161,6 @@ LatLngBounds MapSnapshotter::Impl::getRegion() const {
 }
 
 MapSnapshotter::MapSnapshotter(FileSource* fileSource,
-                               std::shared_ptr<Scheduler> scheduler,
                                const std::pair<bool, std::string> style,
                                const Size& size,
                                const float pixelRatio,
@@ -173,7 +168,7 @@ MapSnapshotter::MapSnapshotter(FileSource* fileSource,
                                const optional<LatLngBounds> region,
                                const optional<std::string> programCacheDir,
                                const optional<std::string> localFontFamily)
-   : impl(std::make_unique<util::Thread<MapSnapshotter::Impl>>("Map Snapshotter", fileSource, std::move(scheduler), style, size, pixelRatio, cameraOptions, region, programCacheDir, localFontFamily)) {
+   : impl(std::make_unique<util::Thread<MapSnapshotter::Impl>>("Map Snapshotter", fileSource, style, size, pixelRatio, cameraOptions, region, programCacheDir, localFontFamily)) {
 }
 
 MapSnapshotter::~MapSnapshotter() = default;
diff --git a/platform/default/src/mbgl/util/default_thread_pool.cpp b/platform/default/src/mbgl/platform/thread_pool.cpp
index d3950bb8aa..d213aab71c 100644
--- a/platform/default/src/mbgl/util/default_thread_pool.cpp
+++ b/platform/default/src/mbgl/platform/thread_pool.cpp
@@ -1,12 +1,33 @@
-#include <mbgl/util/default_thread_pool.hpp>
+#include <mbgl/actor/scheduler.hpp>
 #include <mbgl/actor/mailbox.hpp>
 #include <mbgl/util/platform.hpp>
 #include <mbgl/util/string.hpp>
 
+#include <condition_variable>
+#include <mutex>
+#include <queue>
+#include <thread>
+
 namespace mbgl {
 
+class ThreadPool final : public Scheduler {
+public:
+    explicit ThreadPool(std::size_t count);
+    ~ThreadPool() override;
+
+    void schedule(std::weak_ptr<Mailbox>) override;
+
+private:
+    std::vector<std::thread> threads;
+    std::queue<std::weak_ptr<Mailbox>> queue;
+    std::mutex mutex;
+    std::condition_variable cv;
+    bool terminate { false };
+};
+
 ThreadPool::ThreadPool(std::size_t count) {
     threads.reserve(count);
+
     for (std::size_t i = 0; i < count; ++i) {
         threads.emplace_back([this, i]() {
             platform::setCurrentThreadName(std::string{ "Worker " } + util::toString(i + 1));
@@ -54,4 +75,9 @@ void ThreadPool::schedule(std::weak_ptr<Mailbox> mailbox) {
     cv.notify_one();
 }
 
+Scheduler& Scheduler::GetBackground() {
+    static std::unique_ptr<ThreadPool> pool(new ThreadPool(4));
+    return *pool;
+}
+
 } // namespace mbgl
diff --git a/platform/default/src/mbgl/util/shared_thread_pool.cpp b/platform/default/src/mbgl/util/shared_thread_pool.cpp
deleted file mode 100644
index d7facbab94..0000000000
--- a/platform/default/src/mbgl/util/shared_thread_pool.cpp
+++ /dev/null
@@ -1,14 +0,0 @@
-#include <mbgl/util/shared_thread_pool.hpp>
-
-namespace mbgl {
-
-std::shared_ptr<ThreadPool> sharedThreadPool() {
-    static std::weak_ptr<ThreadPool> weak;
-    auto pool = weak.lock();
-    if (!pool) {
-        weak = pool = std::make_shared<ThreadPool>(4);
-    }
-    return pool;
-}
-
-} // namespace mbgl